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Thread: A nerdy treatise on adventure bike screens: noise, buffeting, turbulence and what-not

  1. #1
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    A nerdy treatise on adventure bike screens: noise, buffeting, turbulence and what-not

    Warning: Long-ish!

    Technical/scientific corrections - welcome. Exhortations to man up, grow a pair and embrace the tinnitus - taken as read.

    TL;DR Noise/buffeting with your adv bike screen? Try a flyscreen.

    Browsing any adventure bike forum, it looks like screens are a common issue for many adv riders, which seems to be confirmed by the healthy trade in aftermarket offerings. It's of particular interest to me, as I've owned an 1100 & 1200 GS, and now a KTM 1050, and in each case, the standard screen was so noisy that it severely limited the bike's utility for fast cruising & distance riding.

    The problem is of course, aerodynamics. Or more specifically in the case of adv-style bikes; lack of aerodynamics, due to the barn-door class format of upright stance, large frontal area, and OEM screens which for all their claims of aerodynamic effectiveness and wind tunnel-guided design, are essentially still just a small plank placed between the rider and the wind.

    Of all the subspecies of motorcycle design (nakeds, cruisers, sportsbikes, sports-tourers etc.) adv bikes present the biggest challenge for managing airflow: they need to protect the rider from windblast in order to offer mile-eating cruising ability, but the format doesn't allow for the hunched posture and streamlined teardrop bodywork which gives sportsbikes & traditional sports-tourers the ideal high-speed aerodynamic shape, nor the huge fairings and windscreens of tourers, which simply displace the problem outside of the space that the rider physically occupies. Naked bikes put the rider in undisturbed air - there's still the sound of airflow, but it's smooth, consistent, and less fatiguing than the roaring & buffeting that so many adv bikers experience.

    Adv bike screens suffer from two principle aerodynamic problems:

    1. Turbulence
    This is the breaking up of smooth airflow into confused, 'dirty' airflow when it hits the vertical slab of plastic in front of your clocks. The airflow is energised & scattered in all directions when it impacts the screen, and the edges of the screen create vortices & eddies, like a rock in the middle of a smooth-flowing river. Instead of smooth & consistent wind, the rider is impacted by chaotic airflow arriving in various directions, densities, and velocities. It gets worse as you go faster, because your forward speed is converted into more energy as your screen impacts the air harder.

    2. Cavitation
    As you ride along, your screen is pushing air out of the way. This increases the air pressure in front of the screen, and reduces the air pressure behind it, creating a partial vacuum. This pressure difference causes air to flow around the edges of the screen to fill the vacuum, which generates a shock wave, which you hear as a thump, and feel, like a rubber mallet tapping your bonce. This process repeats cyclically, a few times per second. (Open your car window at 50+ mph to experience the same effect). As with turbulence, the effect increases with speed. It's also the reason why full-dress tourer riders experience the hand of God pushing them in the back when they raise the screen all the way up.

    A simple solution is to design venting into the screen, in the form of gaps or holes somewhere along the screen's surface, to allow the pressure on each side of the screen to equalise without the violence of air being sucked around the edges of the screen. The downside of this solution is that any perforations in the screen will generate turbulence of their own at their edges, albeit less dramatically than the edge of the screen. Another alternative is to mount the screen outboard of the bodywork, on spacers or brackets, allowing airflow underneath the screen to equalise the pressure with smoother-flowing air from head-on.

    So what are the options?

    Live with it
    If you are the perfect averagely-sized human that the bike was designed for, you may have no problems at all with your bike's standard OEM screen. If not, you might invest in some custom earplugs, although that won't stop your head being jiggled by any buffeting. Or you can just avoid riding above 50 mph as much as possible.

    Go large
    The full-dress tourer approach. Get a screen so big, that all the turbulence is directed out beyond the space you occupy. Cavitation effects may actually be worse with this option though unless the screen is vented somewhere, and you may well end up looking through the screen instead of over it. Another important consideration is the effect that a disproportionately large screen will have on your bike's aesthetic appeal, although if you've experienced the misery of a few hours' slogging on the motorway with a noisy screen, you may not care.

    Go naked
    Who will rid me of this turbulent screen? You, in a few minutes, with some tools. The bike will probably look a bit crashed, but your helmet will be in smooth, clean air. As with any naked bike, your chest will also act as a huge meat airbrake, and speeds above 60 mph will require you to expend greater effort in hanging on to your bike.

    Go small
    A counterintuitive, and consequently much underrated solution (imo) is a smaller screen - often referred to as a flyscreen, sport screen, or OEM screen + Dremel/bandsaw. The principle here is to lower the stream of dirty airflow below the level of your helmet, and reduce cavitation by the simple expedient of generating a smaller vacuum behind the screen. Your helmet will be fully in the wind, but the airflow will be smooth, and the aerodynamic drag of your helmet is far less than that of your chest, which is still protected from the windblast by the screen. There'll still be less noise, because you don't have ears in your chest. Plus the solid, centralised mass of your torso can tolerate turbulence far better than the pumpkin-on-a-spindle of your head. As a final note, small screens tend to have less aesthetic impact, and may even make your bike look a bit more hardcore adventurer/street moto.

    Get scientific - Venturi/airstream enfarklements
    This solution involves mounting an additional screen on the screen, either as a deflector/spoiler (Puig, TT, Wunderlich et al.) or an entire screen assembly (MRA Vario, Givi Airflow). The principle here is to create a jet of smooth air which deflects the worst of the dirty air away from your body & head. In effect, a tall screen without the extra hideousness & vision distortion of additional perspex acreage. The gap between the screens creates a Venturi: the constriction of flow increases the speed of the airflow, whilst simultaneously reducing its pressure (which scavenges more airflow by vacuum suction than by wind pressure alone)

    More adjustment
    Many of the adv bike manufacturers celebrate the adjustability of their screens, but it generally seems to be a token, feature-touting, box-ticking effort, rather than an effectively engineered solution to the problem of their customers' diversity in height, width and volume.

    Considering the range of fixed-size replacement screens available, full-range enhanced adjustability is surprisingly under-represented as a solution. Tobinators and Madstad Robobrackets are the only ones I know of.


    Findings & recommendations
    This is a personal view based on the practical experience gleaned from pouring scadloads of my hard-earned cash into the money pit of aftermarket adventure bike screens.

    - Try a smaller screen, not a bigger one
    Sport/flyscreens are usually at the cheaper end of the replacement screen market, reduce noise & buffeting at cruising speeds, and may even enhance the appearance of your bike rather than disfigure it. It may not be protective enough if you like to cruise for extended stints at illegal speeds.

    - Get something adjustable
    Replacing the standard screen with a bigger "touring" screen may gain you an improvement, but unless your dealer or a friend kindly lets you try before you buy, you risk dumping a load of cash on something that's still disappointing. Adjustability increases the chances that you'll be able to tune the screen to your specific size & shape or alternatively, find an acceptable compromise between noise/buffeting, and protection from windblast.

    Clip-on/bolt-on spoilers may be small, but I've found that they make a useful difference thanks to their range of adjustment in both height & angle. In addition to simply physically blocking the wind by adding height to an existing screen, they also create a modest Venturi effect to smooth the airflow at the top of the screen.

    Givi Airflow and Madstad replacement screens offer a properly useful, stepless range of adjustment, especially compared to the paltry adjustment ranges of OEM screens. The Givi has a Venturi gap for pressure equalisation and airstream deflection, and the Madstad is mounted outboard for pressure equalisation, as well as being adjustable in 3 axes. They are designed for function over form however, so check that you can live with the looks.


    My solutions (6' tall, 34" inside leg, short body, big head)

    R1200GS Hexhead (2007)
    Givi Airflow + winglets: brilliant - even better than the R1150 RT which I had a brief & unsatisfactory affair with. Screen worked best at half-height, which didn't look too hideous, and didn't intrude into field of view. Engine audible at 70 mph, and practical ugliness is subsumed into the clunky overall GS aesthetic. Downsides? Visor steams up easily.

    KTM 1050 Adventure
    KTM short 'R' screen plus Puig clip-on deflector: way better than OEM screen, both functionally and visually. Comfortable at 70 mph, compared to unbearable with OEM. Downsides? Helmet collects an epoxy spackling of insect splats.

  2. #2
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    Really interesting post. I've recently joined the GS brigade and am toying with the aftermarket screen quagmire. It certainly raises some interesting talking points snd things to try out.

    Peter

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